A very sensitive fluorogenic calpain substrate, Abs/Em=501/527 nm. In general, Rh110-based protease substrates are much more sensitive than the analogous AMC or AFC-based substrate. This enhanced sensitivity might be attributed to the greater fluorescence of the enzymatic product. The colorless and nonfluorescent (Z-Ala-Ala)2Rh110 is hydrolyzed to the highly fluorescent Rhodamine 110, which exhibits excellent spectral properties that match the optimal detection window of most fluorescence instruments. Alternatively, (Z-Ala-Ala)2Rh110 can also be used to detect calpain in a chromogenic mode since the enzymatic product (Rhodamine 110) exhibits a large extinction coefficient (close to 100,000 cm-1mol-1). After cleavage, Abs(max) is at 497 nm and Em(max) is at 527 nm.